A current scheme for making changes to electronic substrates, such as ceramic, glass or glass-ceramic substrates, uses redundant connection pads to add new connections or delete existing connections. The original connections from the integrated circuit chips, hereinafter referred to as IC's, to the electronic substrate are normally made using solder or braze connections between the IC's and the connection pads located beneath the IC's. Surplus connection pads, each of which is connected to an original connection pad, are located around the periphery of the IC's. New or additional connections may be made between redundant connection pads, for example, by wire bonding, staple bonding or by the use of decals. Connections may be deleted by severing the line connecting a redundant pad to an original pad. If, during electronic substrate rework, an IC must be removed and replaced, solder and braze removal is limited to the area that was directly below the IC, leaving the peripheral connections undisturbed.
However, because space on a substrate is both scarce and valuable, it is desirable to use the available space between the original connection pads beneath the chip itself, rather than, or in addition to, the peripheral area, to make such circuit changes. In such a scheme it becomes necessary to remove solder or braze from the original connection pads without disturbing the immediately adjacent areas where fragile connections, such as wire bonds, staple bonds or decals, may be present. Additionally, if the redundant connection pads are deposited with a layer of gold, it may be desirable to prevent solder or braze from depositing over the gold or dissolving the gold during rework.
The ability to remove solder or braze from a surface is similarly desirable for making changes to any electronic circuit substrate which uses surface-mounted IC's, components or wiring, for example, epoxy-based electronic cards.
A variety of devices for removal of solder or braze from electronic substrates have been previously disclosed. The disclosed devices can be classified by two approximate categories, the first being flat-surface blocks for the solder or braze removal, and the second being wicks for the removal of solder or braze from the surface of an electronic substrate.
Within the flat-surface block category, for example, is disclosed a tinned porous copper slug prepared by powder metallurgical methods for module reworking, "Use of a Tinned Copper Slug for Module Reworking", IBM Technical Disclosure Bulletin, Vol. 24, No. 7A, p. 3481 (December 1981). A tinned copper slug is applied to a substrate and the tinned copper slug absorbs melted solder by capillary action as the substrate is heated.
Also disclosed in the flat-surface block category is a porous copper block having solid plated studs, "Chip Rework on Multilayer Ceramic Recess", IBM Technical Disclosure Bulletin, Vol. 27, No. 10B, p. 6344 (March 1985). A porous copper block having solid plated studs is placed on a substrate, such that the solid plated studs enter substrate recesses where excess solder is located. As the copper block is pushed down, the solid plated studs are forced into the recesses, the excess solder is then pushed outwardly and it is then absorbed by the base portion of the porous copper block.
Several variations of wicks capable of absorbing solder by capillary action have been disclosed. For example, U.S. Pat. No. 4,164,606 discloses a wick formed by braiding strands of copper that have been individually coated with solder. U.S. Pat. No. 4,323,631 discloses a wick formed from strands of non-metallic material which are coated with metal and flux to render the fibers more capable of wetting. U.S. Pat. No. 4,416,408 discloses a wick comprised of an open mesh of strands.
Several methods for making porous metal devices have been previously described. Disclosed in IBM Technical Disclosure Bulletin, Vol. 25, No. 5, p. 2285 (October 1982), is a method for making porous metal devices which employs a single sintering step. A mold is filled with copper powder, vibrated, covered and heated to a temperature of 890 degrees Celsius in an atmosphere of disassociated ammonia. After cooling, the copper devices, which have undergone approximately 10 percent shrinkage, are removed from the mold. U.K. Patent No. 661,780 discloses a strikingly similar process for making porous metal molds to be used to make pottery and wood fiber egg trays.
Australian Patent No. 162,811 also discloses a method for making porous metal devices. Spherical particles are used which have a metal coating that is softer than the core metal and preferably alloyable with the core metal. The particles are compressed into a briquette, such that the softer coating is distorted and holds adjacent particles together. The briquette is then sintered such that the softer coating alloys with the harder core.
U.S. Pat. No. 2,219,423 discloses a method for forming porous metal articles of a complex shape. Flat briquettes are placed over a mold having a complex shape. The mold cover which may have a shape that matches the mold is placed over the briquette before sintering. During sintering the weight of the mold cover presses the briquette until it conforms to the shape of the mold.
Brofman, et al., U.S. Pat. Nos. 5,219,520 and 5,284,286, issued to the assignee of the instant patent application, disclose a porous metal block that is used to selectively remove solder or braze from a semiconductor substrate. The porous metal block comprises a plurality of protrusions which absorb the solder or braze through capillary action. Disclosed according to the invention is a porous metal block for selectively removing solder or braze from a substrate. The porous metal block comprises at least one protrusion capable of absorbing solder or braze through capillarity. Also disclosed is a sintering process for making the block employing two sintering steps. A mold is filled with metal powder and presintered (partially sintered) to form a presintered porous metal block strong enough for subsequent processing. The presintered porous metal block is then ejected from the mold and fully sintered at a higher temperature.
Miller, IBM Technical Disclosure Bulletin, Vol. 12, No. 4, Page 548 (September 1969), discloses a method for removal of excess solder by using a jet of hot oil. The solder is melted and is carried away by the hot oil.
Schink, et al., IBM Technical Disclosure Bulletin, Vol. 18, No. 5, Pages 1384-85 (October 1975), disclose the cleaning of solder pads prior to a rework operation. The solder to be removed is heated by a hot gas which is directed towards the solder pad site and the exiting hot gas heats and dislodges the solder and the solder debris become a part of the jet stream and is thus removed.
Ward, IBM Technical Disclosure Bulletin, Vol. 19, No. 7, Page 2476 (December 1976), discloses a method of removing solder on a substrate by immersing the substrate in a solder alloy which will dissolve C4 tin lead connections.
The possible rework methods can be grouped into 3 categories: Solder wicking methods that absorb liquid solder into a porous medium or similar material, a dissolution method in which a module is immersed into a liquid solder bath that removes the high lead components, and a fluid impingement method such as a modified solder leveling technique.
The wicking method involves two steps, removing the solder ball or columns and then dressing the off-eutectic residue using an absorbing material. But because the eutectic temperature associated with the molten portion of the residue is below the liquidus temperature of the disbursed solid residue, wicking devices such as porous copper blocks act as separating devices, absorbing the liquid portion but leaving behind the solid proeutectic Pb-rich phase components. Another disadvantage to this two-step process is that two thermal excursions are required, one to remove the solder balls or columns and the second to dress the residue.
Dissolution methods only require one thermal excursion. They utilize a solder bath, usually of eutectic composition, in which the solid Pb-rich phase components of the off-eutectic solder joints dissolves into the solution. As the bath is used over time, the composition of the bath changes, increasing in lead content and so too correspondingly the liquidus temperature. These Pb additions beyond the eutectic composition result in a Pb-rich phase precipitating out of the liquid. Thus, the solder bath must be changed from time to time.
Fluid impingement methods such as a hot gas solder leveling technique would work well to remove the solder balls or columns and dress the residue on the pads. But, the high-lead components will stay solid and blow around on the ceramic substrate. This can result in solder being imbedded into the ceramic posing a metal migration concern. Even if a fluid or flux is used to coat the ceramic substrate first, particle imbedding can occur due to the high velocities of the air stream impinging on the substrate.